Sanding system controls

[ledwardz]

I probably know less about a sanding system, but assume that a train is used to put sand on the rails to increase adhesion in poor conditions. I also assume the sand needs to be delivered evenly at a set volume/weight per length of rail. Tell me if this is wrong as I imagine a train with hopper(s) feeding sand to nozzle(s) through some kind of variable valves.

It would be desirable for the sanding train to be able to travel (while sanding) in between timetabled traffic to keep the railway operating as normally as possible. That means that the train will be affected by Speed restrictions, red signals and generally not travel at a nice slow constant speed. A sanding system needs to be controlled to respond to the train speed in order to keep delivering an even coating (i.e. to provide constant sand per meter, by variable flow per minute)
It needs to switch off when stopped, it may need to change the angle of delivery nozzle(s) when the track has a curve or camber, it may need to warn when a hopper is blocked, nearly empty and so on. Failing to do this could mean erratic braking adhesion leading to possible SPADs; erratic TC behaviour which could release route locking or allow conflicting routes to set or clear signals in error; there may be issues with traction return currents owing to high resistance; and probably a load of other things I can't imagine. If uncontrolled, and the sand heaped up while stopped at a signal maybe a derailment could result.

Excess sand missing the rail may also affect track bed/ballast conditions, I don't know, but building up the ballast level was a factor in an incident where a signal post rusted through and the signal fell over (encouraged damp conditions and prevented inspection). Sand on the trackbed could make conditions underfoot even more difficult for those working on the line (maintainers for example), thus being a factor in slips/trips/falls accidents.
What about 3rd rail/4th rail electrified areas - presumably the sand must stay clear of the electrification rail or it will do damage.
What about a sanding train going over a Level Crossing - might you want to prevent the road from being sanded?

These are the sort of things you might be thinking of in terms of controlling a sanding system, and the consequences for signal engineers.
You might think about possible mitigation to protect the signalling system such as requiring sequential operation of track circuits, but is this disproportionate if the sanding is infrequent and/or well controlled.?

Hi Dorothy Pipet.

Really good reply and simple to understand, thanks very much. i will commit this to memory for the exam in hope a sanding question comes up.....

could you elaborate on the sequential operation of track circuits?

google tells me its track circuits with an interface to the locking system that applices sequential logic so that when an out of sequence detection occurs the area is 'quarantined'

cheers,

Lee.